16 research outputs found
Simulating Charge Injection and Dynamics in Microscale Organic Field-Effect Transistors
Monte Carlo simulations were used to investigate the
carrier dynamics in realistic, finite-sized, small-molecule, organic
field-effect transistors (OFETs) within the first few nanoseconds
of device turn-on as well as when the system equilibrates. The results
show that the device current exhibits large magnitude oscillations
(64 ± 27 nA) during device turn-on if the initial configuration
assumed no carriers in the device (i.e., carriers only arrive through
injection from the source electrode). After equilibration (125 ns),
the current continues to oscillate, however, at lower magnitude (64 ±
2 nA), even if the initial configuration assumed randomly placed charges.
Fourier transforms of device current as a function of simulation time
show that these oscillations occur at well-defined device geometry-dependent
frequencies, independent of initial configuration of the system. Examination
of the carrier lifetimes and path lengths, which were found to vary
nonlinearly with device length, are used to argue that the oscillations
are the result of the charge injection procedure, which assumed a
constant probability event. The results suggest that carriers travel
in waves in realistically finite-sized devices and that carrier lifetime
and path length vary nonlinearly by device geometry. Alternating current
studies of OFETs may be useful in confirming these findings
Piezoelectric Effects of Applied Electric Fields on Hydrogen-Bond Interactions: First-Principles Electronic Structure Investigation of Weak Electrostatic Interactions
The piezoelectric
properties of 2-methyl-4-nitroaniline crystals
were explored qualitatively and quantitatively using an electrostatically
embedded many-body (EE-MB) expansion scheme for the correlation energies
of a system of monomers within the crystal. The results demonstrate
that hydrogen bonding is an inherently piezoelectric interaction,
deforming in response to the electrostatic environment. We obtain
piezo-coefficients in excellent agreement with the experimental values.
This approach reduces computational cost and reproduces the total
resolution of the identity (RI)-Møller–Plesset second-order
perturbation theory (RI-MP2) energy for the system to within 1.3 Ă—
10<sup>–5</sup>%. Furthermore, the results suggest novel ways
to self-assemble piezoelectric solids and suggest that accurate treatment
of hydrogen bonds requires precise electrostatic evaluation. Considering
the ubiquity of hydrogen bonds across chemistry, materials, and biology,
a new electromechanical view of these interactions is required
Charge Transport in Imperfect Organic Field Effect Transistors: Effects of Charge Traps
Monte Carlo simulations were used to study the effects
of explicit charge traps on charge transport in small-molecule organic
field effect transistors. The results show that the source-drain current
decreases as the trap/barrier concentration increases, reaches a minimum
around 30/70%, and increases as the concentration reaches 100%, regardless
of the trap/barrier distribution. Greater current is predicted for
heterogeneous trap distributions than for homogeneous trap distributions,
due to wider conduction pathways that allow for more charge carriers
to reach the drain electrode. Also, the distributions of distances
and potential energy between charge carriers and trap sites were shown
to depend on the heterogeneity of the traps and device geometry and,
in most cases, are non-Gaussian in shape, due to electrostatic effects
between charged traps, unlike previous assumptions. For some ranges
of heterogeneity, these densities of states exhibit exponential tails.
These results suggest that more experimental work is needed to gain
insight into the energetic density of states under operating conditions
in electronic devices made from mixed films of organic semiconductors,
such as solar cells
Charge Transport in Imperfect Organic Field Effect Transistors: Effects of Charge Traps
Monte Carlo simulations were used to study the effects
of explicit charge traps on charge transport in small-molecule organic
field effect transistors. The results show that the source-drain current
decreases as the trap/barrier concentration increases, reaches a minimum
around 30/70%, and increases as the concentration reaches 100%, regardless
of the trap/barrier distribution. Greater current is predicted for
heterogeneous trap distributions than for homogeneous trap distributions,
due to wider conduction pathways that allow for more charge carriers
to reach the drain electrode. Also, the distributions of distances
and potential energy between charge carriers and trap sites were shown
to depend on the heterogeneity of the traps and device geometry and,
in most cases, are non-Gaussian in shape, due to electrostatic effects
between charged traps, unlike previous assumptions. For some ranges
of heterogeneity, these densities of states exhibit exponential tails.
These results suggest that more experimental work is needed to gain
insight into the energetic density of states under operating conditions
in electronic devices made from mixed films of organic semiconductors,
such as solar cells
Piezoelectric Effects of Applied Electric Fields on Hydrogen-Bond Interactions: First-Principles Electronic Structure Investigation of Weak Electrostatic Interactions
The piezoelectric
properties of 2-methyl-4-nitroaniline crystals
were explored qualitatively and quantitatively using an electrostatically
embedded many-body (EE-MB) expansion scheme for the correlation energies
of a system of monomers within the crystal. The results demonstrate
that hydrogen bonding is an inherently piezoelectric interaction,
deforming in response to the electrostatic environment. We obtain
piezo-coefficients in excellent agreement with the experimental values.
This approach reduces computational cost and reproduces the total
resolution of the identity (RI)-Møller–Plesset second-order
perturbation theory (RI-MP2) energy for the system to within 1.3 Ă—
10<sup>–5</sup>%. Furthermore, the results suggest novel ways
to self-assemble piezoelectric solids and suggest that accurate treatment
of hydrogen bonds requires precise electrostatic evaluation. Considering
the ubiquity of hydrogen bonds across chemistry, materials, and biology,
a new electromechanical view of these interactions is required
Charge Transport in Imperfect Organic Field Effect Transistors: Effects of Charge Traps
Monte Carlo simulations were used to study the effects
of explicit charge traps on charge transport in small-molecule organic
field effect transistors. The results show that the source-drain current
decreases as the trap/barrier concentration increases, reaches a minimum
around 30/70%, and increases as the concentration reaches 100%, regardless
of the trap/barrier distribution. Greater current is predicted for
heterogeneous trap distributions than for homogeneous trap distributions,
due to wider conduction pathways that allow for more charge carriers
to reach the drain electrode. Also, the distributions of distances
and potential energy between charge carriers and trap sites were shown
to depend on the heterogeneity of the traps and device geometry and,
in most cases, are non-Gaussian in shape, due to electrostatic effects
between charged traps, unlike previous assumptions. For some ranges
of heterogeneity, these densities of states exhibit exponential tails.
These results suggest that more experimental work is needed to gain
insight into the energetic density of states under operating conditions
in electronic devices made from mixed films of organic semiconductors,
such as solar cells
Charge Transport in Imperfect Organic Field Effect Transistors: Effects of Charge Traps
Monte Carlo simulations were used to study the effects
of explicit charge traps on charge transport in small-molecule organic
field effect transistors. The results show that the source-drain current
decreases as the trap/barrier concentration increases, reaches a minimum
around 30/70%, and increases as the concentration reaches 100%, regardless
of the trap/barrier distribution. Greater current is predicted for
heterogeneous trap distributions than for homogeneous trap distributions,
due to wider conduction pathways that allow for more charge carriers
to reach the drain electrode. Also, the distributions of distances
and potential energy between charge carriers and trap sites were shown
to depend on the heterogeneity of the traps and device geometry and,
in most cases, are non-Gaussian in shape, due to electrostatic effects
between charged traps, unlike previous assumptions. For some ranges
of heterogeneity, these densities of states exhibit exponential tails.
These results suggest that more experimental work is needed to gain
insight into the energetic density of states under operating conditions
in electronic devices made from mixed films of organic semiconductors,
such as solar cells
Comparing the Experiences of Highly Skilled Labor Migrants in Sweden and Japan : Barriers and Doors to Long-term Settlement
As labor markets become increasingly global, competition among industrialized nations to attract highly skilled workers from abroad has intensified. Spurred by concerns over future economic needs caused by the demographic challenges of an aging population, both Japan and Sweden have joined this global competition. This article examines Japanese and Swedish immigration policies for highly skilled migrants and compares the highly skilled migrants' experiences in the two countries through interviews with these migrants. Despite Japan and Sweden's completely different approaches to immigration itself, both countries' policies, as well as the experiences of the skilled migrants, are strikingly similar. Highly skilled migrants experience language barriers and prejudice in both countries, making it difficult to build social networks with natives. Career development seems to be perceived as a common problem, although less so in Sweden, where labor markets are more flexible. Overall, these issues reduce both Japan's and Sweden's ability to retain skilled migrants. While they share similarities, Sweden's famed work-life balance and gender equality give it an edge in the competition for skilled migrants, which Japan does not share. This comparison identifies which social conditions facilitate or impede skilled migrant settlement
Single-Molecule Piezoelectric Deformation: Rational Design from First-Principles Calculations
Conventional piezoelectric materials
change shape in response to
an applied external electric field, frequently deforming at grain
boundaries in addition to intrinsic unit cell changes. We detail a
computational investigation, using density functional theory (DFT)
calculations of single-molecule piezoelectrics. Rather than deforming
along covalent bond lengths or angles, these molecular springs, derivatives
of [6]Âhelicene and phenanthrene, change conformation in response to
the applied field, up to 15% of the molecular length. A substituted
[6]Âhelicene has a predicted piezoelectric coefficient of 48.8 pm/V,
and one of the phenanthrenes yields a piezoelectric coefficient of
up to 54.3 pm/V, which is significantly higher than polymers such
as polyvinylidine difluoride (PVDF) and comparable to conventional
inorganic materials such as zinc oxide (ZnO). We discuss structural
properties that are found to yield large piezoresponse and hypothetical
target molecules with up to 64% length change and a predicted piezoelectric
coefficient of 272 pm/V. Based on these findings, we believe a new
class of highly responsive piezoelectric materials may be created
from the “bottom up”, yielding immense electromechanical
response
Piezoelectric Hydrogen Bonding: Computational Screening for a Design Rationale
Organic
piezoelectric materials are promising targets in applications
such as energy harvesting or mechanical sensors and actuators. In
a recent paper (Werling, K. A.; et al. <i>J. Phys. Chem. Lett.</i> <b>2013</b>, <i>4</i>, 1365–1370), we have
shown that hydrogen bonding gives rise to a significant piezoelectric
response. In this article, we aim to find organic hydrogen bonded
systems with increased piezo-response by investigating different hydrogen
bonding motifs and by tailoring the hydrogen bond strength via functionalization.
The largest piezo-coefficient of 23 pm/V is found for the nitrobenzene–aniline
dimer. We develop a simple, yet surprisingly accurate rationale to
predict piezo-coefficients based on the zero-field compliance matrix
and dipole derivatives. This rationale increases the speed of first-principles
piezo-coefficient calculations by an order of magnitude. At the same
time, it suggests how to understand and further increase the piezo-response.
Our rationale also explains the remarkably large piezo-response of
150 pm/V and more for another class of systems, the “molecular
springs” (Marvin, C.; et al. <i>J. Phys. Chem. C</i> <b>2013</b>, <i>117</i>, 16783–16790.)